1. Field of the Invention
The present invention relates to a semiconductor manufacturing process, more specifically, to a deep trench structure manufacturing process, which is able to reduce leakage problems, thereby improving the electrical performance of products.
2. Description of the Prior Art
In semiconductor deep trench manufacturing process, with reference to FIG. 1a, a deep trench is formed in a substrate 10 on which a pad oxide layer and a pad nitride layer 11 are formed. Then a thin dielectric layer, preferably a nitride layer of which the material is preferably silicon nitride, is formed to cover the sidewall and bottom of the deep trench. Then, a first conductive layer 13 is deposited in the deep trench. The portion of the nitride layer not covered with the first conductive layer 13, which can be poly-silicon, is removed, and the portion of the nitride layer 12 covered with the first conductive layer 13 is left. As shown in the drawing, when the portion of the nitride layer is removed by etching, the nitride layer 12 is usually etched to a level lower than the top of the first conductive layer 13, so that a gap 15 is formed.
Subsequently, according to the deep trench process of the prior art, a portion of the sidewall of the deep trench not covered with the first conductive layer 13 is oxidized by, for example, thermal oxidation to form an oxide layer 14, as shown in FIG. 1b. For the sake of convenient description, the oxide layer 14 refers to a preliminary oxide layer.
An oxide is formed in the deep trench by chemical vapor deposition (CVD) or any other proper method, and etched by dry etching or any other proper method to form a collar oxide layer 16 on the portion of the sidewall of the deep trench not covered with the first conductive layer 13. Then the deep trench is filled with a second conductive layer 17 upon the first conductive layer 13, as shown in FIG. 1c. However, in practical manufacturing process, the collar oxide layer 16 fails to enter the gap 15. Accordingly, the gap 15 still exists even after the collar oxide layer 16 is formed.
Then, portions of the oxide layers 16 and 14 not covered with the second conductive layer 17 are removed by wet etching or any other proper method. Generally, the oxide layers are etched to a level lower than the top of the second conductive layer 17, as shown in FIG. 1d. 
Finally, the deep trench is filled with a third conductive layer 18 upon the second conductive layer 17, so that the structure shown in FIG. 1e is finished.
The above process has some problems. As stated above, in the deep trench structure, a gap is likely to be formed between the nitride layer 12 and the collar oxide 16, leading to a path for junction leakage, thereby influencing the sub-threshold leakage and accordingly the performance of the entire structure. In addition, since the formation of the preliminary oxide layer 14 is due to the reaction between sidewall of the deep trench and oxygen, and a portion of the preliminary oxide layer is removed in the step of removing oxide as shown in FIG. 1d, the critical dimensions of the opening and the upper portion of the deep trench are increased, so as to influence the aspect ratio of the deep trench.
Therefore, there is a need for a solution to overcome the problems stated above. The present invention satisfies such a need.